These days, more startups are going the route of launching with multiple co-founders and building traction on the side before eliminating the team’s full-time income. While bringing on a co-founder can increase your likelihood of success, there’s no denying that a bad co-founder can quickly sink any side project before it has time to flourish.

So how do you ensure that you pick a reliable cofounder? I had the opportunity to sit down and interview co-founders Jen Kessler and Chiara McPhee of the email marketing platform Bizzy (recently acquired by SendGrid), as well as investor Kat Mañalac of Y Combinator, who recognized their potential and helped them through the accelerator program.

During our interviews, it became clear that a few key qualities can make all the difference in determining whether someone will truly be a reliable co-founder. So which attributes matter most? Let’s take a look.

1. Skills complimentary to yours.

One thing that quickly became apparent in both interviews was the importance of complementary skill sets. Mañalac explained, “You want to work with someone who isn’t a clone of yourself. …Maybe one person is more technical and more product-focused, and the other person is more focused on sales and talking to customers. Ideally you’ll have complimentary skills.”

Kessler further expounded on this when she noted, “When you are a small team with limited resources, you need to make sure that everybody brings something new to the table and has distinct roles and responsibilities. The art and science yin-yang has been a big asset in [my] time working [with McPhee].”

Simply put, your company isn’t going to last very long if you and your co-founder are both programming experts but have zero marketing experience. Your founding team needs to have a broad range of skills to successfully manage the many demands of running your own startup.

2. Niche-specific experience.

An additional area to consider with complimentary skill sets is the importance of at least one of the founders bringing niche-specific experience to help guide the company’s development. As Mañalac noted, this can make a strong impression on investors, because, “when founders are building something to solve a problem they’ve experienced, they’re much more tightly tied to that problem and they understand the problems and user base more intuitively.”